Photolithography techniques are used in the manufacture of integrated circuits. Due to the use of light in the exposure of photo resist, when two devices on the wafer are too close to each other, optical proximity effects occur. Optical proximity effects are due to light diffraction and interference between closely spaced features, resulting in the widths of lines in the lithographic image being affected by other nearby features. The proximity effects affect the process control in the formation of features, e.g., contacts such as gate electrodes and drain/source electrodes.
Double patterning is a technology developed for lithography to enhance feature density. Typically, for forming features of integrated circuits on wafers, lithography technology is used which involves applying a photo resist and defining patterns on the photo resist. The patterns in the patterned photo resist are first defined in a lithography mask, and are implemented either by the transparent portions or by the opaque portions in the lithography mask. The patterns in the photo resist are then transferred to the manufactured features.
With the increasing down-scaling of integrated circuits, the optical proximity effect posts an increasingly greater problem. When two separate features are too close to each other, the space and/or pitch between the features could be beyond the resolution limit of the light source. In accordance with double patterning technology, closely located features are separated into two masks of a same double-patterning mask set, with both masks used to pattern the layer. In each of the double-patterning masks, the distances between features are increased over the distances between features in a single mask, and hence, the resolution limit is overcome.